122 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



projecting anteriorly over the base of the odontoid and terminating behind 

 in a stout rod, which extends well behind the postzygapophyses. The 

 latter structures are narrow, but project far behind the centrum, so that 

 they are visible for their full extent when the vertebra is viewed from the 

 ventral side. The transverse processes are short, slender and imperforate. 

 The anterior opening of the vertebrarterial canal is large and pierces the 

 base of the neural arch in front of the transverse process. From this 

 point the canal runs diagonally upward and backward through the side of 

 the neural arch and opens posteriorly into the neural canal. 



In MacraucJienia the axis differs from that of Theosodon in the follow- 

 ing respects: (i) The centrum is broader and less constricted in the mid- 

 dle. (2) The odontoid process is shorter. (3) The neural spine is higher 

 dorso-ventrally and projects farther in front, not so far behind. (4) The 

 anterior opening of the canal for the vertebral artery is smaller and placed 

 higher up on the pedicel of the neural arch. 



The third cervical is very long and has a centrum which, aside from the 

 anterior cotyles, is very like that of the axis, with a constriction in the 

 middle and broad, depressed faces ; the ventral keel, as in the axis, divides 

 posteriorly into two divergent ridges. The neural canal is small and the 

 zygapophyses are long and narrow, projecting well in front of and behind 

 the centrum. The neural spine is a long ridge, with straight and nearly 

 horizontal border; anteriorly, the spine is quite high, posteriorly, the 

 neural arch rises so as to reduce the height of the spine, which dies away 

 upon the arch in front of the postzygapophyses. The anterior opening of 

 the vertebrarterial canal is at the base of the pedicel of the neural arch, 

 the remainder of the canal being concealed within the arch. The trans- 

 verse processes are short, but much extended in the antero-posterior direc- 

 tion ; each is divided into two portions, a smaller anterior part, which 

 projects in front of the centrum, and a larger posterior part. Both parts 

 have a horizontal direction and lie in nearly the same fore-and-aft plane. 



The fourth cervical differs from the third in several respects. The 

 centrum is less depressed and its faces have more nearly equal transverse 

 and dorso-ventral diameters. The neural spine is much less extended 

 antero-posteriorly, but nothing can be determined as to its height, since it 

 is broken in all of the available specimens ; the zygapophyses are some- 

 what larger and the canal for the vertebral artery has a similar position. 

 On the other hand, the transverse processes are quite different ; the ante- 



